1. Field of the Invention
The present invention relates to gravitational separation of particles and more particularly relates to a device and a process for the gravitational separation of solid particles having density differences.
2.Description of the Related Art
Prior processes and devices for purification of solid particles, for example iron ore, include systems such as set out in Yang, U.S. Pat. 4,592,834, issued Jun. 3, 1986, which is incorporated herein by reference. Prior processes for mechanically separating silica (SiO.sub.2) from iron ore (e.g., magnetic concentrate) at high processing rates have been unable either (1) to reduce silica levels from above 5.5 weight percent based on the total weight of the iron ore to below 5.0 weight percent based on the total weight of the iron ore or (2) to recover iron values in the product more than 95 percent based on the total weight of iron ore in the feed pulp. These problems associated with alleviating in combination (1) low (reduced) silica levels in the final product and (2) high (enhanced) iron recovery levels generally resulted from the inability (or inefficiency) of prior processes to separate out iron fines (particle sizes of smaller than 150 mesh size or 100 microns) from silica fines (smaller than 150 mesh size or 100 microns). Various crude iron ores contain agglomerates of iron rich material and silica rich material, and failure to adequately comminute (crush, powder, pulverize or grind) the iron ore results in inadequate separation of the iron material and the silica material. Consequently, in prior processes carrying a substantial amount of silica along with the iron thereby often resulted in undesirably high (greater than 5 percent by weight) silica impurities in the final iron product. Conversely, excessive comminuting (pulverizing, grinding, powdering, or crushing) can result in high levels of fines (particle sizes of smaller than 150 mesh) which cannot be effectively and efficiently separated via prior processes such as flotation processes or magnetic separation processes.
Traditionally, coal or mineral gravity separation is carried out in a variety of separation devices such as thickeners, cyclones, tables, jigs, spirals, and heavy media separators. These conventional methods depend on size, shape and densities of the particles to be separated as well as fluid dynamic conditions in the separators. The separation efficiency, however, deteriorates as the feed material becomes finer or if particle sizes vary greatly.
Heavy media separation for coal cleaning, for example, is only effective for treating particles coarser than 28 mesh. Even though flotation works on particles sizes less than 28 mesh, flotation cannot be used to reject pyrite particles which tend to coalesce with coal as a froth product due to their similar surface hydrophobicities. In addition, the results of conventional flotation techniques are relatively poor in comparison to density-based coal washability. Additionally, conventional jigging processes have typically experienced instabilities and vorticity in the dense particle media, resulting in undesirable vertical mixing in the media. Furthermore, small particle sizes typically result in undesirably high levels of short circuiting in jigging processes.
Consequently, there is a need for devices and processes which will in combination provide high purity (for example, low silica iron ore) product and will provide high product (for example, iron) recovery levels.